In the capture of imagery using a distributed array of RF antennas, physics generally dictates that the aperture of a sensor must be very large in order to capture imagery with high resolution. Typical solutions to this problem use very large antennas or a steering system to synthetically create a larger aperture. However, it has been shown that sparsely populating an aperture with distributed antennae provides a sensor of acceptable resolution, but requiring significantly reduced size, weight, and power (SWaP). Optoelectronic devices may be used to modulate the captured RF information onto optical carriers. At the receiver, the signals may be combined, and imagery can be reconstructed, e.g., using an infrared camera.
Optical fibers used as data transmission media are typically sensitive to environmental effects, especially vibration. Additionally, the performance of supporting components, such as elements in front-end sensor arrays, may be affected by temperature and humidity. All of these effects can cause the phases of the transmitted signals to drift between channels. In order to focus the array and also compensate for detrimental environmental effects, it may be desirable to synchronize the relative phases of the channels such that they are coherent. Such a system may use a device to measure the phase of each channel and may provide feedback to the system to compensate for the various sources of error.
An interference technique has been used for down-sampling the phases of very high frequency optical carriers in order to focus optically up-converted RF signals in a distributed aperture imager. Off-the-shelf industrial control boards use one ADC per channel to sample an analog waveform, obtaining instantaneous voltage measurements with high quantization and sampling period. Such a phase synchronization solution may use 15 Rack Units (RU) of equipment to synchronize a limited number of optical channels.
When there are hundreds or thousands of elements in the sensor array, the task of phase synchronization becomes even more complex. Using classical techniques to sample the resulting interference patterns with full precision requires an exorbitant amount of bandwidth and electronic components. Further, supporting electronics that use typical analog-to-digital conversion/digital-to-analog conversion (ADC/DAC) industrial control techniques are too large and heavy to be reasonably deployed in a portable system. It may be desirable to overcome these problems, which may allow for field deployment of a system with many more elements and thus better performance than would be otherwise possible.